Current solar cells based on single semiconductor material have an intrinsic efficiency limit of approximately 31%. A primary reason for this limit is that no one material has been found that can perfectly match the broad ranges of solar radiation, which has a usable energy in the photon range of approximately 0.4 to 4 eV. Light with energy below the bandgap of the semiconductor will not be absorbed and converted to electrical power. Light with energy above the bandgap will be absorbed, but electron-hole pairs that are created quickly lose their excess energy above the bandgap in the form of heat. Thus, this energy is not available for conversion to electrical power.
Higher efficiencies were thought to be achievable by using stacks of semiconductor with different band gaps, forming a series of solar cells. The concept is that the higher gap materials convert higher energy photons, allowing lower energy photons to pass down to lower gap materials in the stack. Stacks of two semiconductors, GaInP/GaAs and three semiconductors GaInP/GaAs/Ge have been developed over the last decade, and have the highest efficiency of any solar cell. Because of the lack of appropriate semiconductor materials, attempts to make solar cell stacks with more junctions have actually resulted in lower efficiencies.
Currently most efficient tandem cells use fixed gap combinations, 1.85/1.43 eV for two junction cells and 1.85/1.43/0.7 eV for the three junction cells. The cells take advantage of the relatively good lattice match of Ga0.5In0.5P, GaAs and Ge. However the cells based on these fixed energy gap combinations do not take full advantage of the solar spectrum. There is a need for a solar cell that converts more of the light spectrum into electrical power.